1. Field of the Invention
The present invention relates to a two-band oscillation apparatus to be used in a hand-held telephone set used in common for different two systems.
2. Description of the Related Art
FIG. 3 illustrates a structure of a two-band oscillation apparatus of the related art consisting of a first oscillator 21, a second oscillator 22 and an amplifier 23. The first oscillator 21 is structured to oscillate at a low frequency band (Lf) of about 880 MHz to 940 MHz. The second oscillator 22 is structured to oscillate at a high frequency band (Hf) of about 1805 MHz to 1920 MHz which is higher than the oscillation frequency of the first oscillator 21.
The power source voltage (Vb) is applied to the first oscillator 21 or second oscillator 22 through a first change-over switch 24. The oscillator to which the power source voltage is applied operates to output an oscillation signal. An amplifier 23 amplifies such oscillation signal.
The amplifier 23 is formed as an emitter-grounded tuning amplifier. The power source voltage is impressed to the collector of the amplifying transistor 23a via a choke inductor 23b. The emitter of amplifying transistor is grounded through a bias resistor 23c and a bypass capacitor 23d. Moreover, a series circuit of a tuning capacitor 23e and a switching diode 23f is connected between the collector and ground. The cathode of switching diode 23f is grounded and a switching voltage (Vs) is impressed to the anode thereof via a second change-over switch 25. The second change-over switch 25 is changed over in conjunction with the first change-over switch 24.
When the power source voltage is impressed to the first oscillator 21 with the first change-over switch 24, a switching voltage is impressed to the anode of the switching diode 23f of the amplifier 23 via the second change-over switch 25 to turn ON the switching diode 23f. An oscillation signal of the low frequency band output from the first oscillator 21 is input to the amplifier 23. In the amplifier 23, a parallel tuning circuit is formed of a choke inductor 23b and a tuning capacitor 23e when the switching diode 23f turns ON. The tuning frequency is set almost to be matched with the low frequency band. Therefore, the amplifying characteristic of the amplifier 23 shows the peak amplification degree in the low frequency band as illustrated in A of FIG. 4. The oscillation signal of the low frequency band input to the amplifier 23 is amplified to a large extent. The amplified oscillation signal is output from the collector of amplifying transistor 23a via a DC-cut capacitor 23h.
Conversely, when the power source voltage is impressed to the second oscillator 22 with the first change-over switch 24, a change-over voltage is no longer impressed to the anode of the switching diode 23f of amplifier 23 with the second change-over switch 25 to turn OFF the switching diode 23f. An oscillation signal of the high frequency band output from the second oscillator 22 is then input to the amplifier 23. In the amplifier 23, when the switching diode 23f turns OFF, its terminal capacitance is connected in series to the tuning capacitor 23e. The combining capacitance of these elements is connected in parallel to the choke inductor 23b to form a parallel tuning circuit. The tuning frequency is set almost to be matched with the high frequency band. Therefore, the amplifying characteristic of the amplifier 23 has the peak amplification degree in the high frequency band as illustrated in B of FIG. 4. The oscillation signal of high frequency band input to the amplifier 23 is amplified to a large extent. The amplified oscillation signal is output from the collector of amplifying transistor 23a via the DC-cut capacitor 23h.
As explained above, the two-band oscillating apparatus of the related art forms an amplifier to amplify the oscillating signal of two frequency bands with a tuning amplifier. Moreover the tuning frequency of the tuning circuit of the tuning amplifier is changed over. Therefore, a large number of parts are required to impede reduction in size of the circuit.